1. Field of the Invention
This invention relates to a blood pressure measuring apparatus such as a so-called hematomanometer, tonometer or the like. More particularly, it relates to a blood pressure measuring apparatus having a pressurizing means which is operated by a driving means such as an electric motor.
2. Description of the Prior Art
A blood pressure measuring apparatus which is electronically controlled has a pump for supplying pressurized air to a pressure applying means such as a so-called cuff. The pump is driven by a small electric motor. The motor is controlled by a suitable electronic control means having a CPU. When the pressure of the cuff is insufficient, the pump is operated to restart the pressurization. Hereinafter, such a operation is referred as "repressurization". A conventional blood pressure measuring apparatus often fails to start the repressurization because of the stiffness of the pump.
A blood pressure measuring apparatus of the prior art will be described with reference to FIGS. 2A to 2C. The apparatus shown in FIG. 2A comprises a DC motor 1, a CPU 2, a transistor 3, a pump 4, and resistors R1 and R2. The motor 1 is mechanically connected to the pump 4 via an accentric shaft 5 and an operating rod 6 connected to a diaphragm 4a (FIGS. 2B and 2C). The pump 4 has a check valve (not shown in FIGS. 2A to 2C). When the output signal of the CPU 2 is low and the transistor 3 is off, a voltage V.sub.p is not applied to the motor 1 so that the pump 4 is not operated. When a cuff (not shown here) is to be pressurized, the output signal of the CPU 2 becomes high to turn the transistor 3 on. Then, the pump 4 is driven by the motor 1 to supply compressed air to the cuff. When the pressure of the cuff is increased, the output signal of the CPU 2 becomes low so that the motor 1 stops, resulting in that the diaphragm 4a and the rod 6 are positioned at their highest position, owing to the pressure of the cuff.
If the pressure of the cuff is insufficient, the output signal of the CPU 2 becomes high again so as to start the repressurization. In this case, the motor 1 must rotate the shaft 5 against the pressure in the pump 4. Namely, the shaft 5 must be driven by a force which is greater than the pressure in the pump 4. In order to facilitate the repressurization, the CPU 2 of the improved apparatus outputs a driving signal for repressurization (hereinafter, referred as "repressurization signal") which consists of one short pulse P1 between times t1 and t2 and a long pulse P2 between times t3 and t4, as shown in FIG. 3. Alternatively, the repressurization signal of the CPU 2 may consist of two or more short pulses P1 and a long pulse P2, as shown in FIG. 5.
At the time t1 the CPU 2 outputs the repressurization signal of FIG. 3, and the motor 1 begins to rotate in the forward direction indicated in FIG. 4A, so that the operating rod 6 moves downward. When the force of the motor 1 is inadequate, it fails to make a turn as large as 180 deg., and the repressurization signal falls in this state (time t2), resulting in that the operating rod 6 ceases the downward movement and begins to be forced back by the pressure of the pump 4 (FIG. 4B).
The operating rod 6 thus forced back causes the shaft 5 to rotate in the reverse direction. When the shaft 5 has passed the upper dead point, the repulsive force of the pressure medium (air) of the pump 4 begins to cause the shaft 5 and motor 1 to rotate in the forward direction. Under this state, the repressurization signal becomes high again (time t3). The turning force owing to the pressure medium of the pump 4 cooperates with the turning force of the motor 1 in applying a greater force to the shaft 5 in the forward direction. In other words, the repulsive force of the pump 4 gives an impetus to the shaft 5. Hence, the turning force applied to the shaft 5 surpasses the counter force produced by the pump 4, so that the motor 1 can continue to rotate in the forward direction (FIG. 4C).
If it is not sufficient in producing an ample amount of the impetus force by one pulse P1, the repressurization signal may have two or more short pulses which are produced in series, as shown in FIG. 5.
In the above-described apparatus, the starting properties of the motor under the condition that the cuff has been already pressurized is improved. Even when a small electric motor is used as the motor 1, consequently, it can drive the pump 4 to conduct the repressurization.
The width, duration and number of the short pulse(s) P1 and also the timing of the application of the pulse are suitably set in accordance with many factors such as the size and material of the diaphragm 4a and the degree of eccentricity of the shaft 5, so that the pulse(s) P1 is applied adequately.
However, these factors vary depending upon the size or type of the apparatus, or, in some cases, upon each individual apparatus even when the size or type is same, resulting in that such an apparatus sometimes fails to conduct the repressurization and that the pressure of the cuff cannot be raised to a level for an adequate pressure measurement. In a conventional apparatus, therefore, the timing must be precisely adjusted, which complicates the manufacturing process. The above-described apparatus still remains to be further improved.